A longitudinal clinical trial of contracepting women was conducted to compare the effectiveness of two dosages (100mg or 150mg) of depot-medroxyprogesterone acetate (DMPA) (Machin et al., 1988), an injectable contraceptive. A total of N=1151 women kept a menstrual diary according to whether they had experienced amenorrhea in four successive three-month intervals(binary outcome).

A marginal model for the binary outcome (amenorrhea or no amenorrhea) was fitted. The results are below.